docs: centralize Markdown developer manual

docs/src/mavlink.md

@@ -0,0 +1,20 @@
+# MAVLink and telemetry
+
+Kaptan emits telemetry using MAVLink 2 framing. The production receiver does
+not promote inbound MAVLink frames to authority records:
+
+- MAVLink 1 is discarded.
+- CRC-invalid MAVLink 2 frames are discarded.
+- Signed frames are discarded because no signature verifier or persistent
+  replay state exists.
+- Unsigned MAVLink 2 frames are also discarded; a V2 CRC detects corruption,
+  not authentication.
+
+`TODO(signing)`: any future inbound authority requires verified MAVLink 2
+signing, configured source/target binding, persistent anti-replay state, and a
+provisioning/key-rotation policy as one reviewed feature.
+
+Telemetry transmission uses bounded frame classes. A critical frame may
+replace pending best-effort traffic, but never the DMA-owned active buffer.
+Completion retries preserve ordering and retain a transiently busy frame for a
+later service turn.